The present invention relates to an on-site impact test to measure the natural frequencies of reactor components.
As reactor recirculation pumps rotate, five vanes pass two cutwaters sequentially. Each time a vane passes the cutwater, the sudden change in fluid velocity causes pressure waves to be propagated upstream and downstream. Because the action of the two cutwaters occurs at exactly the same frequency, pressure waves from each cutwater add to form two pressure waves, one traveling upstream, and the other traveling downstream
The frequency of these pressure waves is equal to the pump rotational speed times the number of vanes and is called the vane passing frequency (VPF). If the recirculation pump is operated such that the vane passing frequency coincides with the jet pump sensing line (JPSL) natural frequency, large amplification of sensing line motion will occur. If large amplification of the sensing line motion occurs due to near resonance of the sensing line natural frequency to the vane passing frequency, large vibration stresses are induced in the sensing line and sensing line support bracket welds. These large stresses may cause fatigue usage accumulation, eventually leading to crack initiation and crack propagation.
It is desirable to know the natural frequency of the JPSL accurately so that the plant can operate to avoid the VPF in that region or to install clamps to move the JPSL natural frequency above the maximum VPF.
JPSL susceptible to fatigue failure has been identified with finite element (FE) analyses. The FE model of the JPSL was developed and qualified using test data. The qualified FE model was then used to determine the natural frequencies of the JPSLs. Considering the specified tolerances on the JPSL lengths, calculations are made for each sensing line, one with nominal dimensions, and the others with the largest and shortest unsupported spans considering tolerances. The JPSL natural frequencies are determined using these models. The calculations, however, are time-consuming and cannot predict the natural frequency precisely due to the variation in the as-built lengths and weld sizes of the JPSLs. Sometimes, the lines are bent during construction.